The present invention relates to a booster as may be used in a brake of an automobile, and more particularly, to a booster which allows an increased output to be obtained when a force of depression (input) applied to a brake pedal is equal to or greater than a given value.
A brake booster is known in the art comprising a valve body slidably disposed within a shell, a valve plunger slidably fitted over the valve body and mechanically coupled to an input shaft, an output shaft having a base which opens rearward and which is slidably mounted on the valve body, a reaction disc disposed between the base of the output shaft and the front end face of the valve plunger, a stop fixedly mounted in the base for abutment against the front end face of the reaction disc, an abutment member slidably disposed within the base for abutment against the front end face of the reaction disc, and an urging member for urging the abutment member rearward (see, for example, Japanese Laid-Open Patent Application No. 16,757/1998).
With this arrangement, when the force from the reaction disc which acts upon the abutment member increases above a load to which the urging member is charged to cause the abutment member to move forward, the volume defined within the reaction disc increases, permitting a more gentle rise in the internal pressure of the reaction disc, thus increasing a servo ratio of the booster. Accordingly, if a small servo ratio of the booster during the initial phase of a braking operation, which prevails before the abutment member begins to move, is chosen so that a favorable brake maneuverability is obtained, it is possible to achieve a greater servo ratio of the booster during a later phase of the braking operation after the abutment member has begun to move, thus allowing a braking effort of an increased magnitude to be obtained with a reduced force of depressing a brake pedal.
In the booster disclosed in said Application, the base of the output shaft comprises a bottom which extends radially outward and which is integrally formed with the rear end of an axially extending rod member, a rearwardly extending cylindrical member which is separate from the bottom, and a plurality of bolts connecting between the bottom and the cylindrical member. However, this assembly is disadvantageously expensive because of the cost of the bolts, the cost of machining threaded openings in the bottom to be threadably engaged by the bolts, and the cost of assembling while tightening the bolts.
In addition, in said Application, the urging member comprises either a coiled spring or a Belleville spring, which is used alone, affording a reduced flexibility in the choice of an input applied at a point where the servo ratio of the booster changes upon initiation of movement of the abutment member and of a range in which the servo ratio of the booster can be changed after the abutment movement has begun to move.
In view of the foregoing, it is an object of the present invention to provide a booster which allows both a machining cost and an assembling cost for the base of the output shaft to be reduced as compared with the prior art and which is capable of improving the flexibility in the choice of an input where the servo ratio of the booster changes and a range within which the servo ratio can be changed subsequently.
Specifically, according to a first aspect of the present invention, in a booster arranged in a similar manner as mentioned initially in connection with the conventional booster, the base of the output shaft comprises a holder which is separate from the axially extending rod member, the stop is separate from the holder, the holder is in the form of a bottomed cylinder having a bottom which is secured to the rod member, the stop is secured inside the cylinder, the abutment member and the urging member mentioned above are disposed between the stop and the bottom, and the urging member comprises either a plurality of coiled springs which are concentrically disposed or a single coiled wave spring.
According to a second aspect of the present invention, in the booster as mentioned above in connection with the first aspect of the present invention, the base of the output shaft comprises a bottom which is integrally formed with the axially extending rod member at the rear end thereof, and a cylindrical member which is separate from, but is connected to the bottom, the stop is formed integrally with the base, the stop is formed on the inner peripheral surface of the cylindrical member, the abutment member and the urging member are disposed between the stop and the bottom, and the urging member comprises either a plurality of coiled springs which are concentrically disposed or a single coiled wave spring.
According to a third aspect of the present invention, in the booster constructed in the manner as mentioned above, the base of the output shaft comprises a bottom formed integrally with an axially extending rod member at the rear end thereof, and a cylinder which is connected to the bottom, the stop is formed separately from the base, the stop comprises a body which is disposed for abutment against the front end face of the reaction disc, and a cylinder disposed for abutment against the bottom, the abutment member and the urging member are disposed between the body and the bottom, and the urging member comprises either a plurality of Belleville springs or a single diaphragm spring.
According to the arrangement defined by the first aspect of the present invention, the base of the output shaft comprises a holder including a cylindrical portion which is fixed within the rod member and in which the stop is secured. Accordingly, the machining cost can be reduced in comparison to a conventional arrangement in which the base comprises a bottom, a cylindrical member and bolts which connects therebetween while simultaneously reducing the assembling cost.
Where the plurality of coiled springs are used, the load to which an individual coiled spring is charged and its spring constant can be chosen separately, allowing an improved flexibility to be achieved in the choice of an input where the servo ratio changes and a range in which servo range can be changed subsequently in comparison to the prior art, by choosing a combination of parameters selected for individual coiled springs. Alternatively, where the urging member comprises a coiled wave spring, a greater range of load charged and spring constant selected is allowed in comparison to the use of the coiled springs or Belleville spring if a single spring is used, thus allowing the flexibility in the choice of an input where the servo ratio changes and a range in which the servo ratio can be changed subsequently to be improved in comparison to the prior art. In addition, the coiled wave spring exhibits a greater hysteresis as compared with coiled springs or a Belleville spring, allowing the controllability over a region of greater depression to be further improved.
According to the arrangement defined by the second aspect of the present invention, the base of the output shaft comprises a bottom which is integral with the rod member, and a cylindrical member which is separate from, but which is connected to the bottom. Accordingly, the machining cost as well as the assembling cost can be reduced in comparison to the prior art where the bottom and the cylindrical member are connected together by bolts to form the output shaft.
Where the urging member comprises the plurality of coiled springs, the loads charged and the spring constants of individual coiled springs can be chosen separately, allowing a greater flexibility to be provided in the choice of an input where the servo ratio changes and a range in which the servo ratio can be changed subsequently in comparison to the prior art, as a result of a combination of parameters which are selected for individual coiled springs. Where the urging member comprises a coiled wave spring, if a single spring is used, the load charged and the spring constant can be chosen from a greater range in comparison to the coiled springs or Belleville spring, thus allowing the flexibility in the choice of an input where the servo ratio changes and the range in which the servo ratio can be changed subsequently to be improved in comparison to the prior art. In addition, the coiled wave spring exhibits a greater hysteresis in comparison to the coiled springs and the Belleville spring, allowing the controllability in a region of greater depression to be improved.
With an arrangement defined by the third aspect of the present invention, the base of the output shaft comprises a bottom which is integrally connected to the rod member, and a cylinder in which the stop is disposed. Accordingly, the machining cost as well as the assembling cost can be simultaneously reduced in comparison to the prior art where the bottom and the cylinder are connected together by bolts to define the output shaft.
Where the urging member comprises a plurality of Belleville springs, the loads charged and spring constants of individual Belleville springs can be separately chosen, allowing the flexibility in the choice of an input where the servo ratio changes as well as the range in which the servo ratio can be changed subsequently to be improved in comparison to the prior art, by a combination of parameters which are chosen for individual Belleville springs. Alternatively, where the urging member comprises a diaphragm spring, the load charged and spring constant can be chosen in a greater range by changing a sheet thickness, a number of bulges or a configuration of the bulges. Accordingly, the flexibility in the choice of an input where the servo ratio changes as well as the range in which the servo ratio changes subsequently can be improved in comparison to the prior art.
Above and other objects, features and advantages of the present invention will become apparent from the following detailed description of several embodiments thereof with reference to the drawings.